SSI Configuration

The RMC supports a wide range of SSI transducers. The RMC offers the following configurable settings to accommodate your SSI feedback device:

Support for Binary or Gray Code output formats.
Support for data lengths from 8 to 25 bits.
Support for offsetting the SSI counts. This allows the user to have a set number of counts subtracted from the SSI counts before the counts are used for control. This allows the 16-bit RMC controller to control virtually any 65,536-position unit range on an SSI device.
Support for Balluff’s additional error checking bit.
Support for reversing the drive output in software.

All of these settings, except for the drive-reverse option must be configured up-front from RMCWin. The drive-reverse option is set in the Configuration word axis parameter. Refer to that topic for details.

Selecting SSI Configuration Options

The SSI Configuration dialog is used to configure the RMC to match your SSI feedback device. To display this dialog:

On the Tools menu, click Module Configuration.
In the Slots list, click the SSI module you want to edit.
Click Slot Options.

The dialog displayed will offer two tabs; one for each axis on the SSI module. See the option descriptions below. To change the SSI configuration:

Click the Axis 0 tab.
Set all options to your desired settings.
Click the Axis 1 tab.
Set all options to your desired settings.
Click Update RMC.
The Update Module Configuration dialog box will be displayed to indicate the progress. If the module could not be reset automatically, you may be prompted to reset the module yourself.
In the RMC Configuration dialog box, click Close.

Data Length

SSI devices may be purchased which provide a specific number of bits of data. Some common values include 13, 24, and 25 bits of data. Set the data length field to match your device.

Certain SSI devices have a number of bits but pad it with a zero, which means the total Data Length must be one greater than the manufacturer's stated data length.

If the most significant bit from the SSI feedback is set, the RMC100 will see the value as a negative number and will report a transducer overflow. The counts will go to zero.

Output Format

SSI devices may be purchased which use either Gray code or binary data output. Although the RMC supports both, binary is preferred. Gray code is generally used internally by most absolute rotary encoders, but it offers no advantage when the data is transmitted to the RMC over the SSI interface. More importantly, there are some cases where using Gray code can interfere with transducer error detection. Under Output Format, click the appropriate option to match your device.

Error Method

Choose the method of indicating an overflow error on the transducer.

Bit 20
If bit 20 in the SSI data turns on, or if the SSI data bits are all zeros, the Overflow bit in the Status word will turn on. While the SSI standard does not specify the use of an overflow bit, Balluff does provide an overflow bit (bit 20) on some of its BTL-5 series of transducers. When no magnet is detected, the Balluff transducer sets this bit to indicate an error.
All Zeros
If the SSI data bits are all zeros, the Overflow bit in the Status word will turn on. Most linear SSI transducer manufacturers set the SSI data bits to all zeros if a magnet is not detected.

Count Offset

The RMC imposes a restriction that all positions must be specified as a 16-bit value in position units. A position unit is a user scaled value and therefore is quite flexible. However, there are limitations to this situation. Consider the following example:

A user is using a 2m-long, 5µm-resolution transducer, and wishes a position unit to equal 10 µm. Therefore, the scale is set up so that 1 (one) position unit equals 2 (two) counts.

The user wishes to control the positions beginning at 1500 mm and ending at 2000 mm on the transducer. This would result in position units of 150,000 and 200,000. Because the maximum value for a 16-bit number is 65,535, this would not work. This is where the Count Offset is used.

The number of counts at the starting and ending positions are 300,000 and 400,000. However, because there will be no controlling taking place before 300,000 counts, we can immediately subtract 300,000 counts from the position and act as though the counts received are 0 through 100,000. Therefore, the position units are between 0 and 50,000, which fits within the 16-bit, 65,535 limit.

The above example should describe the need for this field. This field is used in steps of 256 counts. That is, you may subtract off counts of 0, 256, 512, etc., but not values in between. This is not a significant limitation because precise scaling and offsetting can be done through the Scale and Offset axis parameters.

To calculate your desired Count Offset, do the following:

Determine the absolute-minimum count value you will control.
Divide this count by 256.
Discard the fractional portion.
Enter this value into the Count Offset field.

In the example above, the following values would be used.

The absolute-minimum count value to be controlled is 300,000.
Dividing 300,000 by 256 yields 1171.875
Discarding the fractional portion of 1171.875 yields 1171.
Entering 1171 in Count Offset results in a value of 299,776 being used as the actual Count Offset.

Configuring Balluff SSI Transducers

As of this printing, Balluff has two generations of transducers that support SSI: BTL-3 and BTL-5. The part numbers both begin with the format of BTLa-S1bc… (e.g. BTL3-S102), where the lower-case letters are described below.

Note: It is highly recommended that a Synchronized transducer be selected for motion control. BTL5s can be purchased as synchronous by specifying a B in the part number. For example, BTL5-SxxxB-Mxxx-x-xxxx.

a: Generation

3 = Generation 3

5 = Generation 5

This setting does not affect the configuration of the RMC.

b: Code Format

0 = Binary increasing (24 bits)

1 = Gray Code increasing (24 bits)

2 = Binary decreasing (24 bits)

3 = Gray Code decreasing (24 bits)

6 = Binary increasing (25 bits)

7 = Gray Code increasing (25 bits)

8 = Binary decreasing (25 bits)

9 = Gray Code decreasing (25 bits)

Use the Binary/Gray Code and Data Length fields in the SSI Configuration dialog to match the code format of the BTL transducer. The increasing/decreasing option indicates whether the counts increase or decrease away from the head of the transducer. This does not affect the SSI Configuration dialog, but will need to be taken into account when setting the Scale.

c: System Resolution

1 = 1 µm

2 = 5 µm

3 = 10 µm

4 = 20 µm

5 = 40 µm

7 = 2 µm

This setting does not affect the SSI Configuration dialog, but must be taken into account when setting the Scale.

Configuring MTS SSI Transducers

MTS’s Temposonics® III family of transducers uses SSI. These transducers may be purchased with a number of options. These options are indicated by the last seven digits of the part number, in the format of Sabcdef (e.g. RH-T-0360U-RG0-1-S2B1102), where the lower-case letters each indicate an option, as described below:

a: Data Length

1 = 25 bits - Set the Data Length field to 25.

2 = 24 bits - Set the Data Length field to 24.

b: Output Format

B = Binary - Select the Binary output-format option.

G = Gray Code - Select the Gray Code output-format option.

c: Resolution

1 = 0.005 mm (5 µm)

2 = 0.01 mm (10 µm)

3 = 0.05mm (50 µm)

4 = 0.1 mm (100 µm)

5 = 0.02 mm (20 µm)

6 = 0.002 mm (2 µm)

This setting does not affect the SSI Configuration dialog, but must be taken into account when setting the Scale.

d: Performance

1 = Standard

This setting does not affect the Configuration word.

e, f: Scale Orientation

00 = Forward-acting

01 = Reverse-acting

02 = Forward-acting, Synchronized

This setting does not affect the Configuration word.

Note: It is highly recommended that a Synchronized transducer (Scale Orientation of 02) be selected. This ensures that the time between position samples matches the control loop time of the RMC controller. If the transducer is not synchronized, the sample time may not match and make precise speed control difficult.